Here's a question - does the food that you buy contain genetically modified ingredients? It's hard to know. And today, the Senate failed to pass a provision that would've created voluntary national standards for labeling GMO foods. NPR's Allison Aubrey reports on what this means for food companies and for consumers.

ALLISON AUBREY, BYLINE: When it comes to GMOs in our food supply, opinions often seem more prevalent than facts.

(SOUNDBITE OF TV SHOW, "JIMMY KIMMEL LIVE!")

JIMMY KIMMEL: How many of you do not want GMOs in your body?

(APPLAUSE)

KIMMEL: Yes, OK, a lot of people, all right.

AUBREY: At least that's what comedian Jimmy Kimmel found when his show took a camera to a farmers market.

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UNIDENTIFIED MAN: What is a GMO?

UNIDENTIFIED WOMAN #1: It's genetically - I don't know. What is it?

UNIDENTIFIED MAN: What does GMO stand for?

UNIDENTIFIED WOMAN #2: Oh, man, I don't even remember.

AUBREY: Basically, genetic engineering enables scientists to take useful genes from one organism and transfer them into the genome of another. Now, agribusiness and many scientists support this approach. They argue that this technology can help us stay ahead of potentially devastating pests and provide more nutritious foods.

But critics argue that there are too many unknowns about how genetically engineered crops could influence our health and the environment.

SCOTT FABER: This is the most hotly debated issue in food right now.

AUBREY: That's Scott Faber, the executive director of the Just Label It campaign. His coalition, which includes environmentalists, chefs and celebrities, has been pushing for GMO labeling. And he says today's action is actually a win.

The legislation that the Senate shot down would have left it largely up to food companies to adopt voluntary labeling standards, but Faber says GMO labeling should be mandatory.

FABER: Consumers should have the right to choose. They should have the right to know what's in their food and be trusted to make their own choices. That's what this really boils down to. This is really about transparency.

AUBREY: Today's action in the Senate could bring the state of Vermont one step closer to enacting its own law - scheduled to take effect in July - that will require mandatory labeling. Some food industry groups have been trying to prevent the state law from taking effect because they're concerned about what precedent this would set.

The Corn Refiners Association says mandatory GMO labeling could make Americans' grocery bills go way up. John Dunham is an analyst who worked with the corn refiners to assess the potential economic impacts.

JOHN DUNHAM: Some mandatory GMO label could significantly increase costs for consumers for their food products.

AUBREY: Dunham says the cost of creating labels is not the issue. But if the labels meant that more consumers start avoiding foods that contain GMO ingredients, then manufacturers would likely to reformulate.

DUNHAM: To switch products from the GMO to the non-GMO-based ingredient, we're talking about anywhere between hundreds or even thousands of dollars per family.

AUBREY: It's unclear what happens next. The Senate could go back to the drawing board and craft a compromise bill, but some food companies, such as Campbell's Soup, have already said they support mandatory GMO labeling. Food industry analyst Jack Russo says these companies are following the lead of their customers.

JACK RUSSO: No doubt about it. Consumers are pushing for more transparency